topic Re: Loft issue - rails do not intersect all profiles. in Fusion Support Forum

Loft issue - rails do not intersect all profiles.

Anonymous — Sun, 22 Jul 2018 15:46:29 GMT

I have been working on my model for over 2 weeks and I have spent half of that time trying to get loft to work. I keep getting "rails do not intersect all profiles". I have reached a point where I am basically ready to give up because I can't keep wasting my time screwing around with one thing.

So I selected these three profiles and they work fine but I also need to include two rails.

So I select the bottom edge as a rail and it works just fine.

But now when I select the top rail, it just gives me the error.

This obviously means that something is not lining up but I can't figure out what or why.

What makes this so insane is that when I zoom in as far as possible, all points on the top rail (rail 2) line up perfectly while the bottom rail (rail 1) has a point that does not even come close to the line and it works just fine.

Is this a bug?

Regards,

Karl

Re: Loft issue - rails do not intersect all profiles.

jhackney1972 — Sun, 22 Jul 2018 18:31:09 GMT

Take a look at this blog article and see if it gives you some help with your issue.

Re: Loft issue - rails do not intersect all profiles.

jeff_strater — Sun, 22 Jul 2018 18:48:51 GMT

@Anonymous, I did a couple of quick measurements of your design, and you can see that there is distance between the top rail and two of your profiles:

Which are the sources of the errors in loft. When using rails, you have to make sure that the intersection is exact. That usually involves using the Sketch "Project Intersect" command to get the precise point of intersection between the rail and the profile, or vice versa.

I did not try to fix up your sketches, but you should be able to edit either the profile or the rail sketch and use "Project Intersect" to build that precise intersection.

Here is a post I did a while back on this topic: introduction-to-loft-using-rail-curves-in-fusion, that starts out pretty basic, but does cover this topic, as well.

Re: Loft issue - rails do not intersect all profiles.

TrippyLighting — Mon, 23 Jul 2018 01:30:13 GMT

While Fusion 360 might create some form of surface, your model has plenty of problems with curvature which creates problems lofting. Using all these rails is certainly not going to result in success.

When creating curves for lofting don't use connected arcs and lines but contiguous splines and use as few spline points as you can get away with and work with the spline handles. Also, some of the segmented curves you used are not properly constrained e.g tangent or continuous constraints are missing

This is abut as good was I could get it with the geometry in the model, but if it would me I'd start from absolute scratch using a spline only approach. You second sketch already has curvature continuity problems. then you build a surface from it, which picks up these continuity problems etc.

The opening defined by the existing surfaces and the sketch outline was filled with a patch.

Re: Loft issue - rails do not intersect all profiles.

Anonymous — Mon, 23 Jul 2018 13:01:25 GMT

I have been using the "project, intersect" and I still end up with lines that do not properly intersect each other even if it looks like they do. I have made sure that I only draw lines, splines that connect to sketch points and somehow I still manage to end up with some lines, splines that do not precisely intersect each other even after adding constraints like tangency etc.

I can only imagine that when spliting faces, generated edges and points are somehow out of alignment or something and when later making a sketch using those points I end up with lines, splines that do not actually intersect.

And as can be seen from your pictures there is for example a 1.3458E-04mm (0.00013458mm) cap between the edge and curve which is ridicilous.

Re: Loft issue - rails do not intersect all profiles.

TrippyLighting — Mon, 23 Jul 2018 13:11:54 GMT

@Anonymous wrote:

And as can be seen from your pictures there is for example a 1.3458E-04mm (0.00013458mm) cap between the edge and curve which is ridicilous.

You used analytical curves (arcs , lines ) to create surfaces. These surfaces are NURBS surfaces created by NURBS curves. So in oder for your lofts to succeed Fusion 360 has to approximate these analytical curves with NURBS curves, which results in deviations. That is simply how the math works! There is no reason that you need to be more precise than less than 1/10000 of a millimeter for creating surfaces.

Re: Loft issue - rails do not intersect all profiles.

Anonymous — Mon, 23 Jul 2018 13:47:54 GMT

So basically avoid intersecting with nurbs and geometry because it is not accurate?

Re: Loft issue - rails do not intersect all profiles.

TrippyLighting — Mon, 23 Jul 2018 14:08:30 GMT

No, that's not what I am saying in my post.

What I am saying in my posts is that:

Start from scratch using splines for everything to avoid curvature problems. Don't use curves pieced together from lines, arcs and splines.
When using Splines, use as few spline points are possible and work with the spline handles.
Use the tangent (G1) or even the continuity (G2) constraint to constrain splines starts/ends to projected, or intersected curves.
Your model is symmetric across a plane, so only model half of it.
Use helper/guide surfaces so boundary start/end/ conditions conditions such as tangency(G1) or curvature continuity (G2) can be applied.
Use rails very sparingly, again, less is more when lofting.
The patch command can be very helpful in creating certain shapes, such as the brim. While no weights can be applied boundary conditions can be applied there as well.
This should actually be lofting tip #1. Check your curvature and use the curvature comb every time you create a spline or curve and most definitely on any edge that's been created by trimming with a curve or surface.

All these tips above will keep precision in check. In general, however, a deviation of less then 1 thousands of millimeter for surfacing is perfectly fine. Once you have created the surface es you'll have to stitch them. There is a stitching tolerance and the default of 1mm is rather useless. If you lower this to 0.01 mm and it complains about gaps larger than that you might want to check why.

If it does not, you're good to go!